This invention relates to the production of hollow titanium alloy articles, particularly alpha+beta and near alpha titanium alloy articles.
Recent developments in the design of high bypass gas turbine engines have increased the need for the production of lighter, yet stiffer rotating components, such as fan blades and compressor blades. Hollow titanium alloy parts are the prime choice due to their high strength-to-weight ratio and high fatigue resistance. However, the manufacture of hollow titanium alloy components presents several problems.
The most common method for fabricating aerodynamic blades and vanes is by forging solid blanks, followed by machining to achieve the desired shape and contours. While net precision forgings may be produced, these require the use of special alloys known in the art, but the latter are not as efficient as the wrought alloys. In the case of larger vanes, builtup brazed assemblies are typically produced. Each of these present methods is relatively costly and produces structures which are heavier than desirable.
Conn, U.S. Pat. No. 3,936,920, describes the fabrication of an aerodynamic blade or vane comprising an internally stiffened shell structured panel and a root fitting. Fabrication of this blade comprises fabrication of a panel blank composed of top and bottom face sheets diffusion bonded to a honeycomb core. The panel blank is rough trimmed to size, then shaped, by crushing, to an initial aerodynamic shape. The shaped panel blank is tack welded to the root fitting to assure that the parts will maintain their spatial relationship during the die loading step. The panel/root assembly is then vacuum die pressed to diffusion bond the leading and trailing edges and to mechanically interlock the panel to the root fitting. Following the forming and bonding step, the blade requires removal of surplus flashing and, possibly, machining. Conn, U.S. Pat. No. 4,043,498, describes fabrication of a shaped panel blank. Both of these methods require a plurality of component parts and considerable handling of the various components. Both methods are replete with opportunity for contamination of the component parts, which is very adverse to diffusion bonding.
Hollow components can be produced by superplastic forming/diffusion bonding (SPF/DB) of two or more segments in such manner that will produce a hollow internal cavity with optional internal webbing or reinforcement. One way to produce such segments is by investment casting. However, SPF/DB of cast titanium alloy structures is not generally possible due to the coarse microstructure of the as-cast segments. The as-cast microstructure of alpha+beta titanium alloys consists of coarse transformed beta structure. It typically exhibits large beta grains separated by grain boundary alpha phase and colonies of similarly aligned and crystallographically-oriented alpha plates within the beta grains. SPF/DB requires fine two-phase microstructure.
Accordingly, it is an object of this invention to provide a method for producing hollow alpha+beta and near-alpha titanium alloy articles.
Other objects and advantages of the invention will be apparent to those skilled in the art.